Several types of display devices are known which are switchable between a public display mode and a private display mode, with varying degrees of additional cost over a standard display, ease of use and strength of privacy performance.
Devices incorporating such displays include, for example, mobile phones, tablet and laptop computers, desktop monitors, Automatic Teller Machines (ATMs) and Electronic Point of Sale (EPOS) equipment. Such devices can also be beneficial in situations where it is distracting and therefore unsafe for certain viewers (for example drivers or those operating heavy machinery) to be able to see certain images at certain times, for example an in-car television screen while the car is in motion.
Image processing methods exist for producing a privacy effect in liquid crystal displays (LCDs) which, when in the private mode manipulate the image data in a manner dependent on a second, masking, image, and therefore cause that masking image to be perceived by the off-axis viewer when the modified image is displayed. Examples of such image processing methods are given in Powell et al., GB2428152A1, published on Jan. 17, 2007; Broughton et al., WO2009110128A1, published on Sep. 11, 2009; Broughton et al., WO2011034209, published on Mar. 24, 2011; and Broughton et al., WO201134208, published on Mar. 24, 2011. These methods provide an electronically switchable public/private display with no additional optical elements required, minimal additional cost, and satisfactory privacy performance. These methods all utilise the limited resolution of the human visual system by redistributing the luminance produced to the on-axis viewer by a group of neighbouring pixels within that group while maintaining the same overall luminance produced by the group as a whole.
In both WO2011034209 and WO2011034208, it is described how increasing the size of the group of pixels within which luminance is redistributed increases the maximum contrast of the masking image seen by the off-axis viewer. This is also illustrated in FIGS. 3 and 4 of the current application, which show the available off-axis luminance values as a function of on-axis luminance for methods using groups of two and four pixels respectively. WO2011034209 describes how the main image may be adaptively compressed according to its content so as to improve the privacy strength for particular image content.
However, while the off-axis image quality and therefore privacy strength may be increased by these methods, in none of the method described is the range of off-axis luminance values maximised for all on-axis luminance values. Also, for the fixed pattern of brighter and darker pixel produced by these methods in the resulting private mode image relative to the native display resolution, pre-processing of the main input image is required to minimise the appearance of resolution loss. This additional step increases the complexity and memory requirement of the process as a whole.
A method to produce a multi-view effect on a standard LCD using error diffusion based image processing methods, in which the pixel data values are optimised towards satisfying two or more images and two or more different viewing angles simultaneously, is described in Thompson et al., U.S. Pat. No. 6,870,643 issued on Mar. 10, 2002. However, the method of U.S. Pat. No. 6,870,643 requires a more complex error diffusion calculation using two or more weighted error values for the multiple images, and optimising simultaneously for them both. Another method to produce a similar effect using spatial multiplexing of the display pixels for the different images and using a reduced contrast range of data values for each set is described in Kim, et al., “Enabling concurrent dual views on common LCD screens”, In Proceedings of the 2012 ACM annual conference on Human Factors in Computing Systems (CHI '12). pp 2175-2184, dated May 5-10, 2012. However, a fixed resolution loss due to the pre-defined multiplexing of the images is incurred with this method, as information for each image is represented by only half the display pixels.